A lamp for a vehicle may include a light emitting element, such as a light emitting diode, whose consumed electric power is lower than that of an electric bulb and whose life is longer than that of the electric bulb. In such a lamp, as the quantity of light emission of the light emitting element is lower than that of the electric bulb, the light source is formed with multiple light emitting elements. For instance, as shown in FIG. 8, in the lamp LP, multiple light emitting diode rows L1, L2 and L3 respectively have a prescribed number of light emitting diodes 21 connected in series. The light emitting diode rows L1, L2 and L3 are connected in parallel with a power terminal TV and an earth terminal TG to emit light. In the lamp having many light emitting diodes as in the light source described above, even when some of the light emitting diodes do not emit light as the result of an abnormality such as a disconnection, the light emission of other light emitting elements can satisfy a luminous intensity distribution standard required for the lamp. However, when the number of abnormal light emitting diodes increases, the luminous intensity distribution standard may not be satisfied. Accordingly, a disconnection detecting circuit needs to be provided for detecting an abnormality of the light emitting diodes, especially, the disconnection of the light emitting diodes.
As shown in FIG. 12, to detect the disconnection of the light emitting diode, a disconnection detecting circuit 22A may be provided for detecting the disconnection of multiple light emitting diode rows L1, L2 and L3, respectively. In the disconnection detecting circuit 22A, a potential change generated when the disconnection is detected is employed. That is, the disconnection detecting circuit 22A includes disconnection detecting parts D1, D2 and D3 for respectively detecting the disconnection of the light emitting diode rows L1, L2 and L3. The disconnection detecting parts D1, D2 and D3 include, respectively, transistors Tr1, Tr2 and Tr3 that turn on and off both end voltages of load resistances RL1, RL2 and RL3 connected in series to the light emitting diode rows L1, L2 and L3 in accordance with base resistances RB1, RB2 and RB3 as inputs. Collector voltages generated by collector resistances R1, R2 and R3 of the transistors Tr1, Tr2 and Tr3 are respectively provided to a deciding circuit 5A of a control unit CNT through signal lines SL1, SL2 and SL3. In this example, the three disconnection detecting parts are provided for the three light emitting diode rows. Accordingly, the disconnection detecting parts D1, D2 and D3 are respectively connected to the deciding circuit 5A through the signal lines SL1, SL2 and SL3. The deciding circuit 5A recognizes the collector voltage when the disconnection arises in the light emitting diode rows and the transistor is turned off through the signal lines SL1, SL2 and SL3, and the deciding circuit compares the recognized collector voltage with a reference voltage to decide whether disconnection of the light emitting diode rows, that is, an abnormality, has occurred.
In this technique, the deciding circuit 5A recognizes the signal line in which the voltage changes so that the deciding circuit 5A can decide whether an abnormality has occurred in the disconnection detecting parts D1, D2 and D3 connected to the signal line, that is, the disconnection of the light emitting diode rows L1, L2 and L3. Further, the deciding circuit recognizes the light emitting row corresponding to the abnormal disconnection detecting part so that the deciding circuit can recognize the number of the light emitting diode rows in which the abnormality arises in the lamp or positions where the light emitting diode rows are arranged in the lamp and properly decide whether or not the lamp satisfies the prescribed luminous intensity distribution standard. However, in this technique, since the signal lines need to be allowed to correspond to the light emitting diode rows and the disconnection detecting parts on a 1-to-1 basis, the number of signal lines needs correspond to the number of light emitting diode rows. Accordingly, the number of wirings for connecting the deciding circuit to the lamp is increased. Moreover, when the lamps in the right and left sides of a motor vehicle are respectively connected to the deciding circuit, the motor vehicle needs two times as many as the number of wirings. Consequently, the number of wires for connecting the lamps to the deciding circuit increases, thus increasing weight, size and cost.
Patent document JP-A-2004-122913 discloses a structure of a disconnection detecting circuit for detecting the disconnection of light emitting diode rows. In particular, that document discloses a structure in which disconnection detecting parts respectively in light emitting diode rows are connected in common to one signal line and connected to a deciding circuit of a control unit. The change of the potential of the signal line is recognized by the deciding circuit to decide whether an abnormality has occurred in the light emitting diode rows. Therefore, one signal line for connecting a lamp to the control unit may be effectively used to reduce a size, weight and a cost.
However, in the technique disclosed in JP-A-2004-122913, as the plurality of disconnection detecting parts in the light emitting diode rows are connected in common to one signal line, the light emitting diode row in which an abnormality arises cannot be identified. That is, in the technique disclosed in JP-A-2004-122913, the change in the potential of the signal line makes it possible to decide the generation of an abnormality in any of the light emitting diode rows. However, regardless of whether the abnormality arises in one of the light emitting diode rows or the abnormality arises multiple light emitting diode rows, the change in the potential of the signal line is constant, so that the specific abnormalities cannot be identified. Further, when the abnormality arises in any of the light emitting diode rows, since the change in potential of the signal line is the same, it cannot be decided where the light emitting diode row in which the abnormality arises is arranged in the lamp. Accordingly, it cannot be decided whether the abnormality is located within a range where the lamp satisfies a luminous intensity distribution standard and functions normally, or whether the abnormality is located within a range where the lamp does not satisfy the luminous intensity distribution standard. Thus, the disconnection detecting circuit provided in the lamp cannot be effectively used.